The specific objectives of this proposal are to characterize the effects of saturated and unsaturated fats on the high density lipoprotein (HDL) of rabbits rendered hypercholesterolemic by dietary or genetic means and then to investigate mechanisms by which the dietary fat may produce alterations in HDL levels and subpopulations via changes in apo A-I metabolism. The objectives of this proposal will be achieved by characterizing the effect of saturated and unsaturated dietary fat on both the total HDL level and on the HDL subpopulation profile as delineated by rate zonal ultracentrifugation and selected-affinity immunosorption. Apo A-I turnover studies will then be done in order to determine whether the dietary fat has a greater effect on apo A-I synthesis or degradation. The results of the turnover study will be used to decide whether to investigate synthesis or degradation of apo A-I in further biochemical studies. Degradation would be studied by determining the major organs involved in degradation by injecting 125I-tyramine-cellobiose-apo A-I and then, assuming the liver is found to be an important site, examining hepatic uptake and degradation in the perfused liver model. On the other hand, synthesis would be studied by using liver perfusion to determine rates of synthesis and secretion, by assessing relative apo A-I mRNA levels in the liver and intestine by in vitro cell-free translation, and by measuring the rate of apo A-I synthesis in isolated sheets of intestinal epithelial cells. After the effect of dietary fat on normal rabbit HDL has been characterized, the effect of saturated or unsaturated fat will be similarly studied in the Watanabe (WHHL) rabbit, a good model of human familial hypercholesterolemia (FH), since this rabbit, like its human counterpart, is reported to have low HDL levels. It may, therefore, be possible to use this rabbit to study the nature of the HDL defect in the human disease. The ultimate goal of these studies is to shed some light on the source of the reputed "HDL protective effect" in the development of atherosclerosis. These studies may even begin to unravel the paradoxical observation that polyunsaturated fats apparently protect against hypercholesterolemia while at the same time lowering HDL levels.